1. Field of Invention
The present disclosure relates generally to vibration isolation, such as vibration isolation for vehicles with structures mounted external to the vehicle body.
2. Background
Mechanical vibration in a vehicle may have a number of unwanted side effects, ranging from passenger discomfort to early mechanical failure. Vibration may be particularly problematic for vehicles having one or more relatively heavy structures mounted externally to the vehicle body.
One example is a fuel tank mounted external to a helicopter fuselage. Vibrational energy generated by the rotor may be coupled to the fuel tank system. Frequencies at or close to resonant frequencies of harmonic modes of the aircraft system may be particularly problematic, since they can lead to large-amplitude oscillation. Additionally, since the mass of the fuel tank system changes during aircraft operation, the resonant frequencies also change.
In order to reduce the amount of vibration caused by this effect, a number of existing vibration isolation systems may be used. In a first example of a vibration isolation system for an externally mounted fuel tank of a helicopter, an isolated fuel cell floor is included inside the fuel tank structure. A large number of small isolators are positioned throughout the floor. The assembly of small vibration isolators acts to isolate the fuel mass from the airframe and prevents the resonance of airframe natural frequencies with rotor harmonics as the fuel weight changes.
This configuration may not be optimal. First, the vibration isolation system is incorporated on the inside of the fuel tank, reducing the available fuel storage volume. Second, the system may be heavy, complex, and difficult to install, maintain, and repair. For example, such a system may use hundreds of small isolators positioned inside the fuel tank. Replacing the small isolators requires accessing the fuel cell floor, removing it, determining the position of the isolator(s) to be replaced, and positioning replacements accurately.
Another configuration uses vibration isolation fittings between a mounting beam and the fuselage. The vibration isolation fittings each comprise a machined arm with weights and springs positioned to substantially minimize vibratory shears imparted to the beam and the fuselage, regardless of fuel weight. One such system is described in U.S. Pat. No. 4,311,213, which is hereby incorporated by reference herein. Although this system may provide good vibration isolation, it is expensive, complex, and heavy.